Spraybar hole position

wha-tah-hey Tue Nov 08, 2016 3:36 pm

Fiddling with a postage stamp and a horseshoe backplate, I noticed the spraybar hole was in a different position on each, neither at 90 deg to the airstream where there's max pressure differential.
This is a definite no-no on larger stunt engines that can be adjusted and I don't see why it wouldn't make a difference in fuel draw with reedies.
I tried to turn the spraybar with a pair of needle nose pliers, but no go.
A forum search returned zilch on "spraybar" so what thoughts might you folks have about this?
Is there a way to reorient the spraybar without destroying it?

NEW222 Tue Nov 08, 2016 4:24 pm

I wish I could answer you but I have read something about the spraybar orientation a while ago and why it is like that somewhere but cannot remember where. Also you can remove the spraybar but believe that you ruin the backplate. Some do it to replace the needle with a fine thread needle. Try searching for something along the lines of Cox fine thread needle. Hope this may help some.

wha-tah-hey Tue Nov 08, 2016 4:44 pm

Tnx for the reply, I tried all phrases I could think of and found only one thread, on R&Ring a Space Bug Jr. insert with a recommendation to heat the assembly in hot water.
I'm assuming these backplate spraybars are molded in (?) and wonder if heating might allow R&R without destroying the backplate?

pkrankow Tue Nov 08, 2016 4:49 pm

The optimum location for the single hole in a spray bar is not dead straight down the center of the venturi for suction. It is some value between "can't see the hole" and "not straight down the center"

The main reasons are there is a substance being introduced into the low pressure pocket, and the hole has real dimensions. Velocity of air is very important which is why dead straight is not the optimum location.

There was some experimentation done either here or Stunthanger. The end takeaway is that if you can't see the spraybar hole from outside the intake it should be fine.

The spray bar pushes out easily from most back plates. The biggest problem is there are grooves from the knurling and repeated removal will wear the grooves out making a loose fit. Feel for the grooves on reassembly, and try to not make new ones, to minimize this. I use a small phillips screwdrive to push the spraybar out.

Phil

crankbndr Tue Nov 08, 2016 4:59 pm

Marks post on changing spraybar with photos

https://www.coxengineforum.com/t6170-modifiying-horseshoe-backplates?highlight=Horseshoe+backplate#77616

And another

https://www.coxengineforum.com/t882-horseshoe-backplate-spraybar-removal?highlight=horseshoe+backplate#12398

wha-tah-hey Tue Nov 08, 2016 7:04 pm

Great info, Phil and Crank! Thanks to you both!
I'll see about repositioning them tomorrow.
And I found why I wasn't getting results on my searches - "Search all my posts" doesn't mean my posts, it means all posts.
Glad to have that sorted out. Maybe I won't need to bug everyone with so many questions in the future.

NEW222 Tue Nov 08, 2016 7:16 pm

Hey there wha-tah-hey. Also, if you did your search on say 'GOOGLE' would also yield more possible results.

wha-tah-hey Tue Nov 08, 2016 7:25 pm

Thanks, New - I do that sometimes - I figured someone on CEF would have been there/done that. I Love This Forum!

wha-tah-hey Fri Nov 11, 2016 12:18 pm

To update, after R&Ring the spraybar and setting the hole 90 deg to the airflow, the engine gained 400 rpm.
I won't argue it's due to the new position but that sure didn't hurt it. I'll reset the other too - it's almost 180 deg to airflow.

Oldenginerod Fri Nov 11, 2016 8:43 pm

wha-tah-hey wrote:To update, after R&Ring the spraybar and setting the hole 90 deg to the airflow, the engine gained 400 rpm.
I won't argue it's due to the new position but that sure didn't hurt it. I'll reset the other too - it's almost 180 deg to airflow.

I'm not quite getting your figures. At 180 degrees to the airflow, the spray hole will be facing directly at you as you look into the venturi. No way that'll draw any fuel. At 90 degrees means the the hole points directly to the side of the venturi. This would be a high pressure point, also affecting fuel draw. I've always pointed the spray hole directly down the hole, which makes sense to me because the centre point "behind" the spray bar should have the highest pressure depression, creating the greatest draw. Having said that, I can see some advantage in having the hole off-centre some to put the fuel delivery out into a cleaner airflow, aiding atomization. In his ebay review, Warren Leadbeatter suggests 15 degrees.
http://reviews.ebay.ca/Cox-Surestart-Hop-up_W0QQugidZ10000000005390559

I came across an interesting arrangemet on some Gilberts, which have three different spray bar designes. Some have two holes at 180 degrees to each other. Here you have no choice but to put the holes at 90 degrees to the airflow, each side of the venturi, so it must work.

Rod.

wha-tah-hey Fri Nov 11, 2016 9:53 pm

Oldenginerod wrote:

At 180 degrees to the airflow, the spray hole will be facing directly at you as you look into the venturi. No way that'll draw any fuel.

>>This is what I would call 0 deg - facing the filter screen. Right, no fuel draw at all.<<

At 90 degrees means the the hole points directly to the side of the venturi. This would be a high pressure point, also affecting fuel draw.

>>Because of Venturi effect, this position (or 270 deg) has the GREATEST fuel draw due to LOWEST pressure due to FASTEST airspeed through the MOST RESTRICTED space.<<

I've always pointed the spray hole directly down the hole, which makes sense to me because the centre point "behind" the spray bar should have the highest pressure depression, creating the greatest draw.

>>This is what I call 180 deg. - facing the reed. This position creates minimal fuel draw.<<

Having said that, I can see some advantage in having the hole off-centre some to put the fuel delivery out into a cleaner airflow, aiding atomization. In his ebay review, Warren Leadbeatter suggests 15 degrees.
http://reviews.ebay.ca/Cox-Surestart-Hop-up_W0QQugidZ10000000005390559

>>With a single hole as Cox uses, 15ish deg TOWARD the reed may in fact be absolute best. Since a repositioning is sorta "by guess" in any case, "a little bit toward the reed" is probably as good as it gets.<<

I came across an interesting arrangemet on some Gilberts, which have three different spray bar designes. Some have two holes at 180 degrees to each other. Here you have no choice but to put the holes at 90 degrees to the airflow, each side of the venturi, so it must work.

>>This is how the 2-hole spraybar in a Fox, McCoy and many other RV engines is correctly positioned to maximize fuel draw.<<

Rod.

KariFS Sat Nov 12, 2016 2:20 am

It's called "Bernoulli's Principle". Counter-intuitive to me and many others but it does make sense if you think about it. That's also the reason our model airplanes fly Smile

On my Medallion, I set the hole straight towards the engine, but now I think I'll rotate it 60 degrees (the Medallion has a hexagonal spraybar end, hence 90 degrees is not possible)

Spraybar hole position Image17

Interesting subject Smile

wha-tah-hey Sat Nov 12, 2016 7:50 am

Yes, Bernoulli not Venturi. Thank you for the correction and the diagram, Kari.

KariFS Sat Nov 12, 2016 10:26 am

Terry, I didn't mean to correct you, my intention was just to explain it further, not to come across as a smart-donkey Smile

Venturi effect is actually a correct name for the same phenomenom. Now that I think about it, Bernoulli's equation presumes that the density of the fluid stays constant (or does it?) which is not necessary the case when the fluid in question is air... Huh...

I guess I'll need to look into it further, but abstract thinking is hard for me right now as I just had a very spicy pizza and a big glass of beer. Anyway, placing the spray orifice to where the air flow is fastest, should improve the fuel flow as that's where the greatest vacuum is.

wha-tah-hey Sat Nov 12, 2016 11:01 am

Please rest easy, Kari - I took your post as nothing more than a clarification on your part, and a welcome one at that.
Physics classes were a looooong time ago and those two are so closely related I'd forgotten the details, and to clarify fully (after a quick Google refresher Very Happy

) the Venturi effect is an application of Bernoulli's Principle.
Peace and love, brother. Beer Cheers

roddie Sat Nov 12, 2016 12:41 pm

Don't forget.. the spray-hole in a Cox tank-backplate is a non-adjustable 90 degrees to the venturi. I realize that it's a little different than a tube running through the center of an orifice..

FWIW.. I just checked all of my Cox horseshoe-style backplates and a postage-stamp, to find that the spraybar holes are all dead-nuts on center when looking through the front reed-seat side. Same for the Killer Bee. None of them were even slightly off-center. I checked 8 separate backplates in all. I would think that Cox Engineers found that position to be optimal. Imagine being the person at Cox Hobbies with that job? I have to wonder if it was part of an injection-molding process.. that pre-aligned the NVA in a fixture? I doubt that the NVA's were individually pressed-in.

wha-tah-hey Sat Nov 12, 2016 1:35 pm

They're almost surely machine installed and not only the knurling but the ease of removal seems to indicate pressed in rather than molded in.
The spraybar itself causes a restriction on either side and therefore lowest pressure areas that should be the best places for the hole.
Maybe someone with mechanical engineering or physics background can jump in and explain why Cox might want the hole at what seems to be a poor location.

pkrankow Sat Nov 12, 2016 8:15 pm

The spray bars were in fact installed by hand with simple tooling (arbor press?) and not automated. The people assembling the engines were generally not operating the engines.

Phil

wha-tah-hey Sat Nov 12, 2016 9:54 pm

OK, thanks for the info, Phil.
It may well just be the luck of the draw then as to how the spraybar is positioned, unrelated to proper running.

KariFS Sun Nov 13, 2016 2:50 am

Well, here's what NASA's rocket surgeons have come up with:

Spraybar hole position Image18

Case #3 shows an oscillating airflow, and that is the case with our engines. The airflow starts, stops, bounces back etc, so maybe the best place for vacuum is not so easy to predict. It also varies according to engine speed, ambient temperature and probably a lot of other things.

wha-tah-hey Sun Nov 13, 2016 9:12 am

Guys, the term "fluid dynamics" is just about the extent of my knowledge of fluid dynamics but I think the significant thing is that fuel is sucked in by lower pressure (venturi effect) and/or pushed in by higher pressure (atmospheric in the tank), therefore maximum flow AND steadiest flow (for a consistent needle) will be at the point of maximum constriction and lowest pressure - at 90/170 deg to airflow path. I suspect this also gives best atomization of the fuel. Other factors (inertia, viscosity, friction, I don't know what all) may cause the optimum position to have that 15 deg "offset" mentioned earlier, but in any case having the hole facing the reed is a poor location and an easy fix. Two Cents

Oldenginerod Sun Nov 13, 2016 3:46 pm

wha-tah-hey wrote:... in any case having the hole facing the reed is a poor location and an easy fix.

I disagree. If it was a poor location why do most manufacturers (that I know of) assemble their engines that way? I know all the Enyas I have are installed pointing straight down. The press-in spray bar in all my OK Cub engines seems to point straight in.

The point of lowest pressure, hence greatest depression (suction) has to be just at the point following the restriction where the venturi is the widest. If the spray bar has the hole anywhere other than at the closest point to the vacuum source, then that point hasn't yet allowed the airflow to expand back out to the maximum diameter of the intake. This isn't a "scientific" explanation, just me trying to use some of the logic stored in my brain.

As mentioned earlier, off-set may aid atomization and improve fuel delivery & performance, but the lowest pressure point for maximum "suck" would have to be central behind the restriction (spray bar) in my understanding of aerodynamics. Then, I'm happy to be proven wrong. lol!

wha-tah-hey Sun Nov 13, 2016 4:50 pm

Oldenginerod wrote:
The point of lowest pressure, hence greatest depression (suction) has to be just at the point following the restriction where the venturi is the widest. If the spray bar has the hole anywhere other than at the closest point to the vacuum source, then that point hasn't yet allowed the airflow to expand back out to the maximum diameter of the intake.

Like you, Rod, I can't give a definitive explanation because I don't have the scientific background and, like you, this is just my understanding from an admittedly feeble grasp of the science involved and, like you, I'm perfectly willing to be proven wrong Very Happy

:
Firstly, the incoming air isn't compressed by the venturi, it speeds up to avoid being compressed (so to speak - air can't really choose one thing over another), so expansion isn't a part of the process. MOF, I think compression at that point would produce higher pressure not lower.
As the piston goes up it creates a partial vacuum in the crankcase. It's this internal low pressure area (in combination with the higher outside atmospheric pressure pushing air in) that creates the airflow through the intake passage. The restriction caused by the spraybar forces the air flowing past to speed up, causing the greatest localized low pressure at that point, with a relatively sharp gradient that also falls off sharply as the air passes, slows down and pressure increases again - so sharply that by the time it's gotten around to the "back" side of the spraybar it's essentially at crankcase pressure. Now, since we're still on the intake/compression stroke, there is still the somewhat lowered crankcase pressure at that point along with the higher atmospheric pressure in the fuel tank to "suck/push" fuel out so the engine can run with that orientation, but the point of greatest depression is not the point on the spraybar facing "downstream", it's at the point of maximum constriction.
As to why some manufacturers may set the spraybar that way - I dunno.
Lordy mercy, I wish a fluid mechanic would ring in here! lol!

**RknRusty** Sun Nov 13, 2016 8:49 pm

I'm currently searching Stunthanger.com for a good discussion on this subject. There are many threads to peruse, so I will post a link to the one I find which contains the largest percentage scientists populating the replies.

Meanwhile, read this:
http://www.nclra.org/TechTopics/NeedleValvePlacement/NeedleValveAlignment.pdf

Now for my own test experience:
I built an .049 engine with a Tee Dee cylinder and a Killer Bee backplate which has a very wide open venturi. Popular opinion was that 15 deg. offset from straight down the throat always produced the highest RPM and easiest starting. So that's the way I put mine together. I can't find my threads from back then with my RPM numbers, but I tried it at ~15deg., 90deg., and straight down the hole. And straight in was about 1000 RPMs faster than any other position... as opposed to popular opinion.
I got outrun by 2,120 RPM from a tanked Babe Bee with a Black Widow cylinder.
Rusty

**RknRusty** Sun Nov 13, 2016 10:08 pm

I didn't find anything in my search except repetitive guessing. But here's my question:
Other than possible ease of cranking, does it really matter?

The needle can be adjusted anywhere from too lean to too rich. The engine runs best, which for a tach race means fastest, with its optimum F/A mixture. If the spray bar hole is moved slightly in any direction, the needle may need to be turned in or out to compensate. So it seems like the same amount of fuel will flow with the same amount of air flow at the same RPM, once the needle is adjusted for the desired speed... :head scratching emoticon:
Rusty

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